This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Air-borne noise or unwanted sound is a side-effect of industrialization and the modern-day lifestyle. It has adverse effects on human health, both direct and indirect. While a long-term exposure to high levels of noise is found to cause auditory loss, increased noise level results in indirect effects, for example, sleep loss or increased blood pressure. Therefore, controlling and reducing noise levels is important. A major component of noise generated by household appliances, road traffic or industrial noise occurs in the frequency band of 20-4000 Hz. Noting that the human audible frequency range is 20 Hz to 20 kHz, this band is at the lower end of the audible frequency range. For purposes of this disclosure, low frequency band is defined to be ranging from 20 Hz to 4000 Hz.
Methods to control noise can be broadly grouped into (a) reducing the noise generated at source, (b) passive noise control, and (c) active noise control. Focusing on the passive control methods, the solutions are mainly based on two mechanisms, (1) reflection and (2) absorption. The solutions based on the reflection mechanism are referred to as sound barrier materials and those based on absorption are called sound absorbing materials. The performance of conventional sound barrier materials is in general governed by their inertia in the low frequency range, stiffness in the high frequency range, and by damping in the intermediate range defined by its characteristic coincidence frequency. The performance of the conventional barrier material in the inertia controlled region becomes poorer as the frequency is reduced. This situation necessitates high mass per unit area for effective noise reduction at low frequencies. For instance, to achieve a noise intensity reduction of 30 dB at 2100 Hz requires 5 kg/m2, while a mass per unit area of 40 kg/m2 is required at 300 Hz for the same level of reduction. This is undesirable as noise control at low frequencies imposes parasitic weight, cost and reduced portability.
Considering the sound absorbing materials, conventionally, porous materials are used to absorb the energy of the incident sound by dissipation into heat through the back and forth motion of the fluid carrying the sound wave in the pores. The challenge here is that these materials require large space to enable sizable energy absorption, particularly in the low frequency range. It was established that for maximum efficiency the porous material should be placed at approximately λ/4 distance from the surface of a backing wall and have a thickness greater than or equal to λ/10 (λ: wavelength of the sound wave of interest). For a sound wave at low frequencies, the wavelength is of the order of meters, and therefore the absorbing material needs large space which is again undesirable.
The design of lightweight passive treatments for noise barrier applications in the low frequency range has been a challenge due to the needed high mass per unit area. Thereby, blocking of low frequency sound has conventionally only been achieved by using relatively high masses, since alternative stiffness-based or dissipation-based solutions are usually ineffective in that frequency range for unsupported, homogeneous panels.
Accordingly, there is an unmet need for noise control solutions that address the challenges of designing lightweight barriers, particularly in low frequency ranges.